1. Field of the Invention
The present invention relates to a method for lapping a workpiece such as a hard and brittle ceramic board, or any workpiece that is not so resistant to application of a point shock or force.
2. Description of the Related Art
Conventionally, when performing the lapping process to this type of hard and brittle workpiece, it is common to use a lapping apparatus called an epicyclic-gear system. The principal component of such an epicyclic-gear system is simplified and shown in FIG. 1. In this lapping apparatus, an epicyclic-gear-shaped carrier 2 holding several workpieces 1 is arranged between a sun gear 3 and a ring gear 4. Lap surface plates 5 and 6 are arranged at the upper surface and the lower surface of the carrier 2. In this state, the sun gear 3 and the ring gear 4 are rotated, injecting the lapping liquid in which grinding particles are mixed, between the lap surface plates 5 and 6 in that condition. The carrier 2 is caused to rotate and revolve thereby. The upper and lower surfaces of the workpiece 1 are polished by the sliding action between the upper and lower lap surface plates 5 and 6 and the workpiece 1.
In addition, the sun gear 3 and the ring gear 4 are respectively connected to the electric motor for driving (neither is illustrated) via a deceleration mechanism and stepless-transmission structure.
However, in the lapping apparatus including the lapping pressure-fluctuation system, because of various relationships, such as the lapping pressure, a movement torque and velocity, and mechanical static-friction power, the pressure-reduction range is limited. Also a workpiece 1 overflows a carrier 2 when the rotational power exceeding the maximum static-friction power is applied when the pressure is reduced too much. This workpiece 1 is pinched between the carrier 2 and the lap surface plate 5, thereby generating breaks, fractures and other damage to the workpiece.
Moreover, since velocity and a torque are raised stepwise as shown in FIG. 8 and lapping is performed when the lapping velocity increase system is adopted, a load that is first applied to the workpiece 1 can be made low. However, the situation cannot sufficiently prevent the lapping damage of a workpiece 1 from being generated as explained below.
That is, many small projections are formed on the surface of the workpiece 1. At the time of lapping, the grinding particles of the lapping liquid cut into these projections. Because the lapping velocity increase system uses a continuous driving, once grinding particles are encroached into the surface of a workpiece 1, the encroached grinding particles are not released. The workpiece 1 is going to rotate further under the condition of having the particles encroached therein, and then a large load is applied locally to a workpiece 1.
Therefore, the lapping damage imparted to the workpiece 1 is very large. Accordingly, generation of micro-cracks and other damage cannot be sufficiently prevented.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for lapping and a lapping apparatus which reliably prevent the generation of lapping damage and eliminates damage to workpieces that are lapped.
According to a preferred embodiment of the present invention, a method for lapping includes the steps of first performing a lapping operation intermittently to a workpiece, and then performing a lapping operation continuously to the workpiece after finishing the intermittent lapping operation.
According to another preferred embodiment of the present invention, a lapping apparatus includes an epicyclic-gear-shaped carrier capable of holding several workpieces, a sun gear and a ring gear which mesh with the carrier, a lap surface plate arranged in the upper portion and the lower portion of the carrier, and an electric motor arranged to rotate at least one of the sun gear and the ring gear, such that the lapping apparatus performs polishing while injecting lapping liquid in which grinding particles are mixed, between a workpiece and the upper and lower lap surface plates. The lapping apparatus also preferably includes a first controller for first-stage driving which performs the intermittent driving of the electric motor for driving at a low torque, a second controller for main driving which performs the continuous driving of the electric motor for driving at a high torque after finishing an intermittent driving at a low torque, and a switch arranged to switch between the first controller for first-stage driving, and a second controller for main driving.
According to yet another preferred embodiment of the present invention, a lapping apparatus includes an epicyclic-gear-shaped carrier capable of holding several workpieces, a sun gear and a ring gear which mesh with the carrier, lap surface plates arranged at the upper portion and the lower portion of the carrier, and an electric motor arranged to rotate at least one of the sun gear and the ring gear, such that the apparatus performs polishing of a workpiece by injecting lapping liquid in which grinding particles are mixed, between a workpiece and the upper and lower lap surface plates, and rotating at least one side of the sun gear and the ring gear.
The lapping apparatus also preferably includes a low torque drive circuit which performs the intermittent driving of at least one of the sun gear and the ring gear by the low torque motor, and a high torque drive circuit which performs the continuous driving of at least one of the sun gear and the ring gear by the high torque motor after finishing an intermittent driving, and a switch that is arranged to switch between the low torque drive circuit and the high torque drive circuit.
According to a method of various preferred embodiments of the present invention, a lapping operation is first performed intermittently on a workpiece. In such an intermittent lapping process, if rotational power is applied, the grinding particles of the lapping liquid will be encroached into the projections which exist on the surface of a workpiece, and a load will become large suddenly. However, the load is released by stopping, before that load becomes excessive. Therefore, generation of the serious lapping damages, such as micro cracks, is reliably prevented. By performing the predetermined number of these intermittent movements, the projections on the surface of the workpiece are removed to some extent, and become smooth. Maximum static-friction power can be made small according to a predetermined value. Furthermore, in the continuous lapping process performed subsequently, the surface condition of a workpiece is to some extent smooth. Thus, the point impact force which acts on a workpiece becomes much weaker. The surface of a workpiece can be polished to a desired smoothness, while preventing any damage to the workpiece, even when the workpiece is lapped continuously.
In addition, the procedure for performing a lapping operation intermittently, may be perform manually or mechanically.
It is preferred that the intermittent lapping operation is performed with a low torque and a low speed. It is also preferred that the continuous lapping operation is performed with a high torque and high speed.
If intermittent lapping is performed with a high speed or high torque, the load applied to the projection of a first stage workpiece which has a large frictional resistance is large and the damage to the workpiece is also large.
On the other hand, when an intermittent lapping operation is performed with a low torque and low speed, and a continuous lapping operation is performed with a high torque and at high speed, the workpiece is polished slowly without applying a large load to the original workpiece having projections, and then the workpiece is polished at a high speed in the stage where the workpiece is smooth to some extent. The elimination of workpiece damage and the improvement in polishing efficiency are achieved.
It is preferred that the intermittent lapping operation is performed while varying the angle range gradually within a small angle range.
That is, at the start of an intermittent lapping operation, the polishing of a projection is performed, alleviating the point impact force that is otherwise imparted to a workpiece by moving in the small angle range, because there are a large number of the projections on the surface of a workpiece. Also, as the number of projections decreases, by extending the angle range, the projection can be made smooth effectively.
It is also preferred to perform the intermittent lapping operation while reducing the welding pressure applied to the workpiece, as compared to the welding pressure applied to the workpiece during a continuous lapping operation.
That is, an intermittent lapping is performed under reduced welding pressure applied to the workpiece, so that only the projection which exists on the surface of a workpiece can be polished with a small welding pressure. Further, generation of breaking or cracking by excessive welding pressure locally acting on a workpiece is reliably prevented. Moreover, since the pressure is reduced only at the intermittent rotation time, rotational power exceeding maximum static-friction power is not applied and a workpiece does not overflow a carrier.
According to another preferred embodiment of the present invention, because the electric motor for driving is made to switch from intermittent driving with a low torque to continuous driving with a high torque, only one electric motor for driving is required and thus, the size and cost of the apparatus can be reduced.
The switch described above may include an inverter for varying the torque and the rotational speed of the motor. Alternatively, a transmission device may be used to vary the torque or the rotational speed of the motor.
Alternatively, a low torque motor and a high torque motor may be provided and switched selectively to switch between the intermittent lapping and the continuous lapping, such that the controller is quite simple.
Other elements, characteristics, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.